Biodegradable polymer materials are widely employed in medical materials, drug delivery systems, environment-matching materials and the like. New additional functions are recently required and various researches are being made. Particularly in polyhydroxyalkanoate represented by polylactic acid, an introduction of a chemically modifiable functional group into the molecule is investigated, and compounds having a carboxyl group or a vinyl group are reported. For example, polymalic acid is known as polyhydroxyalkanoate having a carboxyl group in a side chain. A polymer of polymalic acid is known, by a manner of polymerization, in an α-type represented by a chemical formula (22):
and a β-type represented by a chemical formula (23):

Among these, as polymalic acid of β-type and a copolymer thereof, U.S. Pat. No. 4,265,247 discloses a polymer formed by a ring-opening polymerization of a benzyl ester of β-malolactone represented by a chemical formula (24):

(R24: benzyl group)
Also as a copolymer of α-type polymalic acid and glycolic acid and a copolymer containing other hydroxyalkanoic acids other than glycolic acid, Japanese Patent Application Laid-open No. H02-3415 discloses a copolymer formed by a 6-membered ring diester monomer represented by a chemical formula (25):

(R25 being a lower alkyl group such as a methyl group, an ethyl group, an n-propyl group, an isopropyl group or a t-butyl group, or a benzyl group)
and a glycolide or a lactide which is a cyclic diester, or a lactone which is an intramolecular ring-closing ester of ω-hydroxycarboxylic acid.
Also as a polyhydroxyalkanoate having a carboxyl group in a side chain, Macromolecules 2000, 33(13), 4619-4627 discloses a ring-opening polymerization of 7-oxo-4-oxepanonecarboxylic acid ester to form a polymer having an ester group in a side chain and a hydrogenolysis of the polymer to obtain a polymer having a carboxylic acid in the side chain. Biomacromolecules 2000, 1, 275 discloses reacting poly(ε-caprolactone) with lithium diisopropylamide and then with benzyl chloroformate to obtain a polymer in which a benzyloxycarbonyl group is introduced into an α-methylene group in a carbonyl group present in the main chain of poly(ε-caprolactone). Macromolecular Bioscience 2004, 4, 232 discloses reacting polylactic acid with lithium diisopropylamide and then with benzyl bromoacetate to obtain a polymer in which a (benzyloxycarbonyl)methyl group is introduced into an α-methylene group in the carbonyl group present in the main chain of polylactic acid. As a polyhydroxyalkanoate having a vinyl group in a side chain, Polymeric Materials Science & Engineering 2002, 87,254 discloses a polymer obtained by a ring-opening polymerization of α-allyl(δ-valerolactone). Also as a polyhydroxyalkanoate similarly having a vinyl group in a side chain, Polymer Preprints 2002, 43(2), 727 discloses a polymer formed by a ring-opening polymerization of 3,6-diallyl-1,4-dioxane-2,5-dione which is a 6-membered ring diester monomer.
Also there is reported a polymer having novel functions by introducing a structure providing a functionality into a polyhydroxyalkanoate in which a chemically modifiable functional group is introduced as described above. International Journal of Biological Macromolecules 25 (1999) 265 (non-patent literature 6) discloses a ring-opening polymerization of a cyclic dimer of α-malic acid and glycolic acid to obtain an α-malic acid-glycolic acid copolymer, and to unprotect the obtained polymer to obtain a polyester having a carboxyl group in a side chain. The carboxyl group of the side chain is chemically modified with a tripeptide to obtain a polymer, which is reported to provide a satisfactory result in an evaluation of a cell adhesivity.